Offload config save functionality to BTIF thread

Offload config save functionality to btif thread from
timer thread as timer callback thread is critical in
a2dp playback case.

If the timer callback thread is busy in config save due
to IO operations, it may lead to a2dp audio choppy.

Fix to avoid the "bt_config.conf" file corruption from
the file system. This will avoid losing the paired
information in some corner case, such as abrupt power
off and on. This patch will ensure bt_config is saved to
NVRAM.

Bug: 24875861
CRs-Fixed: 953993
Change-Id: I893e9afefa89cbab6e7ddd8835ca77d3e316874c